In situ chemical generator and method

ABSTRACT

Chemical generator and method for generating a chemical species at a point of use such as the chamber of a reactor in which a workpiece such as a semiconductor wafer is to be processed. The species is generated by creating free radicals, and combining the free radicals to form the chemical species at the point of use.

This invention pertains generally to the fabrication of semiconductordevices and, more particularly, to a method and apparatus for generatingimportant chemical species in the deposition, etching, cleaning, andgrowth of various materials and layers.

It is in general an object of the invention to provide a new andimproved chemical generator and method for generating chemical speciesat or near the location where they are to be used.

Another object of the invention is to provide a chemical generator andmethod of the above character which are particularly suitable forgenerating chemical species for use in the fabrication of semiconductordevices.

These and other objects are achieved in accordance with the invention byproviding a chemical generator and method for generating a chemicalspecies at or near a point of use such as the chamber of a reactor inwhich a workpiece such as a semiconductor wafer is to be processed. Thespecies is generated by creating free radicals, and combining the freeradicals to form the chemical species at or near the point of use.

FIG. 1 is a diagrammatic view of one embodiment of an in situ chemicalgenerator incorporating the invention.

FIG. 2 is an enlarged cross-sectional view taken along line 2—2 of FIG.1.

As illustrated in FIG. 1, the chemical generator includes a free radicalsource 11 which has one or more chambers in which free radicals arecreated and delivered for recombination into stable species. In theembodiment illustrated, the source has three chambers which are formedby elongated, concentric tubes 12-14. Those chambers include a firstannular chamber 16 between the outermost tube 12 and the middle tube 13,a second annular chamber 17 between middle tube 13 and the innermosttube 14, and a third chamber 18 inside the innermost tube The tubes arefabricated of a material such as ceramic, quartz or metal.

The number of tubes which are required in the generator is dependentupon the chemical species being generated and the reaction by which itis formed, with a separate chamber usually, but not necessarily, beingprovided for each type of free radical to be used in the process.

Gases or other precursor compounds from which the free radicals areformed are introduced into the chambers from sources 21-23 or by othersuitable means. Such precursors can be in gaseous, liquid and/or solidform, or a combination thereof.

A plasma is formed within the one or more chambers to create the freeradicals, and in the embodiment illustrated, the means for generatingthe plasma includes an induction coil 26 disposed concentrically aboutthe one or more tubes, a radio frequency (RF) power generator 27connected to the coil by a matching network 28, and a Tesla coil 29 forstriking an arc to ignite the plasma. The plasma can, however, be formedby any other suitable means such as RF electrodes or microwaves.

Downstream of the tubes, the free radicals are recombined to form thedesired species. In the embodiment illustrated, recombination takesplace in a chamber 31 which is part of a reactor 32 in which asemiconductor wafer 33 is being processed. Recombination can be promotedby any suitable means such as by cooling 36 and/or by the use of acatalyst 37.

Cooling can be effected in a number of ways, including the circulationof a coolant such as an inert gas, liquid nitrogen, liquid helium orcooled water through tubes or other suitable means in heat exchangerelationship with the reacting gases. It can also be effected by passingthe gases through an expansion nozzle to lower their temperature, or bythe use of either a permanent magnet or an electromagnet to converge andthen subsequently expand the plasma to lower its temperature.

A catalyst can be placed either in the cooling zone or downstream of it.It can, for example, be in the form of a thin film deposited on the wallof a chamber or tube through which the reacting gases pass, a gauzeplaced in the stream of gas, or a packed bed. The important thing isthat the catalyst be situated in such a way that all of the gas is ableto contact its surface and react with it.

If desired, monitoring equipment such as an optical emissionspectrometer can be provided for monitoring parameters such as speciesprofile and steam generation.

In the embodiment illustrated, the chemical generator is part of thereactor, and the species produced is formed in close proximity to thewafer being processed. That is the preferred application of thegenerator, although it can also be used in stand-alone applications aswell. It can be added to existing process reactors as well as beingconstructed as an integral part of new reactors, or as a stand-alonesystem.

The generator can be employed in a wide variety of applications forgenerating different species for use in the fabrication of semiconductordevices, some examples of which are given below.

Oxidation

Steam for use in a wet oxidation process for producing SiO₂ according tothe reaction

Si+H₂O→SiO₂+H₂

can be generated in accordance with the invention by admitting H₂ and O₂into one of the plasma generating chambers. When the plasma is struck,the H₂ and O₂ react to form steam in close proximity to the siliconwafer. If desired, oxygen admitted alone or with N₂ and/or Ar can beused to produce ozone (O₃) to lower the temperature for oxidation and/orimprove device characteristics.

It is known that the use of NO in the oxidation of silicon with O₂ canimprove the device characteristics of a transistor by improving theinterface between silicon and silicon oxide which functions as a barrierto boron. Conventionally, NO is supplied to the reactor chamber from asource such as a cylinder, and since NO is toxic, special precautionsmust be taken to avoid leaks in the gas lines which connect the sourceto the reactor. Also, the purity of the NO gas is a significant factorin the final quality of the interface formed between the silicon and thesilicon oxide, but it is difficult to produce extremely pure NO.

With the invention, highly pure NO can be produced at the point of usethrough the reaction

N₂+O₂→2NO

by admitting N₂ and O₂ to one of the chambers and striking a plasma.When the plasma is struck, the N₂ and O₂ combine to form NO in closeproximity to the wafer. Thus, NO can be produced only when it is needed,and right at the point of use, thereby eliminating the need forexpensive and potentially hazardous gas lines.

NO can also be produced by other reactions such as the cracking of amolecule containing only nitrogen and oxygen, such as N₂O. The NO isproduced by admitting N₂O to the plasma chamber by itself or with O₂. Ifdesired, a gas such as Ar can be used as a carrier gas in order tofacilitate formation of the plasma.

N₂O can be cracked either by itself or with a small amount of O₂ to formNO₂, which then dissociates to NO and O₂. In rapid thermal processingchambers and diffusion furnaces where temperatures are higher than thetemperature for complete dissociation of NO₂ to NO and O₂ (620° C.), theaddition of NO₂ will assist in the oxidation of silicon for gateapplications where it has been found that nitrogen assists as a barrierfor boron diffusion. At temperature below 650° C., a catalyst can beused to promote the conversion of NO₂ to NO and O₂. If desired, nitricacid can be generated by adding water vapor or additional H₂ and O₂ inthe proper proportions.

Similarly, NH₃ and O₂ can be combined in the plasma chamber to produceNO and steam at the point of use through the reaction

NH₃+O₂→NO+H₂O.

By using these two reagent gases, the efficacy of NO in the wetoxidation process can be mimicked.

It is often desired to include chlorine in an oxidation process becauseit has been found to enhance oxidation as well as gettering unwantedforeign contaminants. Using any chlorine source such as TCA or DCE,complete combustion can be achieved in the presence of O₂, yieldingHCl+H₂O+CO₂. Using chlorine alone with H₂ and O₂ will also yield HCl andH₂O.

When TCA or DCE is used in oxidation processes, it is completelyoxidized at temperatures above 700° C. to form HCl and carbon dioxide inreactions such as the following:

C₂H₃Cl₃+2O₂→2CO₂+3HCl

C₂H₂Cl₂+2O₂→2CO₂+2HCl

The HCl is further oxidized in an equilibrium reaction:

4HCl+O₂→2H₂O+Cl₂

Decomposition of various organic chlorides with oxygen at elevatedtemperatures provides chlorine and oxygen-containing reagents forsubsequent reactions in, e.g., silicon processing. Such decomposition isgenerally of the form

C_(x)H_(y)Cl_(y)+xO₂→xCO₂+yHCl,

where x and y are typically 2, 3 or 4.

All of the foregoing reactions can be run under either atmospheric orsubatmospheric conditions, and the products can be generated with orwithout a catalyst such as platinum.

The invention can also be employed in the cleaning of quartz tubes forfurnaces or in the selective etching or stripping of nitride orpolysilicon films from a quartz or silicon oxide layer. This isaccomplished by admitting a reactant containing fluorine and chlorinesuch as a freon gas or liquid, i.e. C_(x)H_(y)F_(z)Cl_(q), where

x=1, 2, . . .

y=0, 1, . . .

z=0, 1, . . .

q=0, 1, . . .

and the amount of fluorine is equal to or greater than the amount ofchlorine. It is also possible to use a mixture of fluorinated gases(e.g., CHF₃, CF₄, etc.) and chlorinated liquids (e.g., CHCl₃, CCL₄,etc.) in a ratio which provides effective stripping of the nitride orpolysilicon layer.

Dielectric Films

Other dielectric films can be formed from appropriate precursor gases.Polysilicon can be formed using SiH₄ and H₂, or silane alone. The silanemay be introduced downstream of the generator to avoid nucleation andparticle formation.

Silicon nitride can be formed by using NH₃ or N₂ with silane (SiH₄) orone of the higher silanes, e.g. Si₂H₆. The silane can be introduceddownstream of the generator to avoid nucleation and particle formation.

In addition to gases, the chemical generator is also capable of usingliquids and solids as starting materials, so that precursors such asTEOS can be used in the formation of conformal coatings. Ozone and TEOShave been found to be an effective mixture for the deposition of uniformlayers.

Metal and Metal Oxide Films

Metal and metal oxide films can be deposited via various precursors inaccordance with the invention. For example, Ta₂O₅ films which are usedextensively in memory devices can be formed by generating a precursorsuch as TaCl₅ via reduction of TaCl₅, followed by oxidation of the TaCl₅to form Ta₂O₅. In a more general sense, the precursor from which theTa₂O₅ is generated can be expressed as TaX_(m), where X is a halogenspecies, and m is the stoichiometric number.

Copper can be deposited as a film or an oxide through the reaction

CuCl₂+H₂→Cu+HCl,

and other metals can be formed in the same way. Instead of a gaseousprecursor, a solid precursor such as Cu or another metal can also beused.

Wafer and Chamber Cleaning

With the invention, organic residue from previous process steps can beeffectively removed by using O₂ to form ozone which is quite effectivein the removal of organic contaminants. In addition, reacting H₂ with anexcess of O₂ will produce steam and O₂ as well as other oxygen radicals,all of which are effective in eliminating organic residue. Thetemperature in the chamber should be below about 700° C. if a wafer ispresent, in order to prevent oxide formation during the cleaningprocess.

Sulfuric acid, nitric acid and hydrofluoric acid for use in generalwafer cleaning are also effectively produced with the invention.Sulfuric acid (H₂SO₄) is generated by reacting either S, SO or SO₂ withH₂ and O₂ in accordance with reaction such as the following:

S+2.5O₂+2H₂→H₂SO₄+H₂O

SO+1.5O₂+H₂→H₂SO₄

SO₂+1.5O₂+2H₂→H₂SO₄+H₂O

then quickly quenching the free radicals thus formed with or without acatalyst.

Nitric acid (HNO₃) is generated by reacting NH₃ with H₂ and O₂, or by areaction such as the following:

N₂+3.5O₂+H₂→2HNO₃+H₂O

NH₃+2O₂→2HNO₃+H₂O

Hydrofluoric acid is generated by co-reacting H₂ and O₂ with a compoundcontaining fluorine such as NF₃ or C_(x)H_(y)F_(z), where

x=1, 2, . . .

y=0, 1, . . .

z=1, 2, . . .

Mixed acids can be generated from a single precursor by reactions suchas the following:

SF₆+4H₂+2O₂→H₂SO₄+6HF

NH₂+H₂+1.5O₂→HNO₃+HF

2NHF+H₂+3O₂→2HNO₃+2HF

NF₃O+2H₂+O₂→HNO₃+3HF

 NF₂Cl+2H₂+1.5O₂→HNO₃+2HF+HCl

N₂F₄+3H₂+3O₂→2HNO₃+4HF

N₂F₄+2H₂+3O₂→2HNO₃+2HF

NF₃+2H₂+1.5O₂→HNO₃+3HF

NF₂+1.5H₂+1.5O₂→HNO₃+2HF

NF+H₂+1.5O₂→HNO₃+HF

NS+1.5H₂+3.5O₂→HNO₃+H₂SO₄

2N₂OF+2H₂+O₂→2HNO₃+2HF

NOF₃+2H₂+O₂→HNO₃+3HF

NOF+H₂+O₂→HNO₃+HF

NOCl+H₂+O₂→HNO₃+HCl

NOBr+H₂+O₂→HNO₃+HBr

NO₂Cl+2H₂+O₂→2HNO₃+HCl

S₂F₁O+7H₂+4O₂→H₂SO₄+10HF

S₂F₂+3H₂+4O₂→H₂SO₄+2HF

SF+1.5H₂+2O₂→H₂SO₄+HF

SF₂+2H₂+2O₂→H₂SO₄+2HF

SF₃+2.5H₂+2O₂→H₂SO₄+3HF

SF₄+3H₂+2O₂→H₂SO₄+4HF

SF₅+3.5H₂+2O₂→H₂SO₄+5HF

SF₆+4H₂+2O₂→H₂SO₄+6HF

SBrF₅+4H₂+2O₂→H₂SO₄+5HF+HBr

S₂Br₂+3H₂+4O₂→2H₂SO₄+2HBr

SBr₂+2H₂+2O₂→H₂SO₄+2HBr

SO₂F₂+2H₂+O₂→H₂SO₄+2HF

SOF₄+3H₂+1.5O₂→H₂SO₄+4HF

SOF₂+2H₂+1.5O₂→H₂SO₄+2HF

SOF+1.5H₂+1.5O₂→H₂SO₄+HF

SO₂ClF+2H₂+O₂→H₂SO₄+HF+HCl

SOCl₂+2H₂+1.5O₂→H₂SO₄+2HCl

SOCl+1.5H₂+1.5O₂→H₂SO₄+HCl

 SOBr₂+2H₂+1.5O₂→H₂SO₄ +2HBrCl

SF₂Cl+2.5H₂+2O₂→H₂SO₄+2HF+HCl

SClF₅+4H₂+2O₂→H₂SO₄+5HF+HCl

SO₂Cl₂+2H₂+O₂→H₂SO₄+2HCl

S₂Cl+2.5H₂+4O₂→2H₂SO₄+HCl

SCl₂+2H₂+2O₂→H₂SO₄+2HCl

These are but a few examples of the many reactions by which mixed acidscan be generated in accordance with the invention. Including more H₂ andO₂ in the reactions will allow steam to be generated in addition to themixtures of acids.

In order to devolitize the various resultant products of the reaction ofHCl, HF, H₂SO₄ or HNO₃, either H₂O or H₂ and O₂ can be co-injected toform steam so that the solvating action of water will disperse insolution in the products. The temperature of the water must be coolenough so that a thin film of water will condense on the wafer surface.Raising the temperature of the water will evaporate the water solution,and spinning the wafer will further assist in the removal process.

Native Oxide Removal

The native oxide which is ever present when a silicon wafer is exposedto the atmosphere can be selectively eliminated by a combination of HFand steam formed by adding a fluorine source such as NF₃ or CF₄ to thereagent gases H₂ and O₂. In order for the native oxide elimination to bemost effective, the reaction chamber should be maintained at a pressurebelow one atmosphere.

Photoresist Stripping

H₂ and O₂ can also be reacted to form steam for use in the stripping ofphotoresist which is commonly used in patterning of silicon wafers inthe manufacture of integrated circuits. In addition, other componentssuch as HF, H₂SO₄ and HNO₃ which are also generated with the inventioncan be used in varying combinations with the steam to effectively removephotoresist from the wafer surface. Hard implanted photoresist as wellas residues in vias can also be removed with steam in combination withthese acids.

SO₃ for use in the stripping of organic photoresist can be generated byadding O₂ to SO₂. Similarly, as discussed above, N₂O can be converted toNO₂, a strong oxidizing agent which can also be used in the stripping ofphotoresist.

Hydrofluoric acid for use in the stripping of photoresist can begenerated in situ in accordance with any of the following reactions:

CF₄+2H₂+O₂→CO₂+4HF

CF₄+1.5O₂+3H₂→CO₂+4HF+H₂O

NF₃+O₂+5H₂→N₂+6HF+2H₂O

It is apparent from the foregoing that a new and improved chemicalgenerator and method have been provided. While only certain presentlypreferred embodiments have been described in detail, as will be apparentto those familiar with the art, certain changes and modifications can bemade without departing from the scope of the invention as defined by thefollowing claims.

What is claimed is:
 1. In a method of generating predetermined chemicalspecies for use in processing semiconductor wafers, the steps of:introducing a plurality of precursor materials into different processingchambers in a chemical generator, forming ionized gas plasmas in theprocessing chambers to create free radicals from the precursormaterials, and combining the free radicals to form the predeterminedchemical species in close proximity to the wafer, the precursormaterials being selected to produce all of the free radicals needed toform the predetermined chemical species.
 2. The method of claim 1wherein the plasma are formed within and between a plurality ofelongated, concentric tubes which define the processing chambers.
 3. Themethod of claim 1 wherein the plasmas are formed in a field created byapplying RF power to an induction coil disposed concentrically aroundthe tubes.
 4. The method of claim 1 wherein the step of combining thefree radicals includes the step of quickly cooling the free radicals topromote formation of the predetermined chemical species.
 5. The methodof claim 1 wherein the step of combining the free radicals includes useof a catalyst which promotes combination of the free radicals andformation of the predetermined chemical species.
 6. In a method ofgenerating a predetermined chemical species, the steps of: introducing aplurality of precursor materials into different processing chambers in achemical generator, forming ionized gas plasmas in the processingchambers to create free radicals from the precursor materials, andcombining the free radicals to form the predetermined chemical species,the precursor materials being selected to produce all of the freeradicals needed to form the predetermined chemical species.
 7. Themethod of claim 6 wherein the plasmas are formed in a plurality ofelongated concentric tubes which define the processing chambers.
 8. Themethod of claim 1 wherein the plasmas are formed in a field created byapplying RF power to an induction coil disposed concentrically about theelongated, concentric tubes.
 9. The method of claim 6 wherein the stepof combining the free radicals includes the step of quickly cooling thefree radicals to promote formation of the predetermined chemicalspecies.
 10. The method of claim 6 wherein the step of combining thefree radicals includes use of a catalyst which promotes combination ofthe free radicals and formation of the predetermined chemical species.11. In a method of generating a predetermined chemical species, thesteps of: introducing a first precursor material into a chamber formedwithin a first elongated tube, introducing a second precursor materialinto a chamber formed between the first elongated tube and a secondelongated tube disposed concentrically of the first, applying RF powerto an induction coil disposed concentrically about the tubes to ionizethe precursor materials passing through the chambers and thereby formgas plasmas containing free radicals, and combining the free radicalsdownstream of the chambers to form the predetermined chemical species.12. In a method of generating a predetermined chemical species, thesteps of: introducing a first precursor material into a first processingchamber, introducing a second precursor material into a secondprocessing chamber, forming an ionized gas plasma in each of theprocessing chambers to produce free radicals from the precursormaterials, and combining the free radicals with each other and with amolecular species to form the predetermined chemical species.
 13. In amethod of generating a predetermined chemical species, the steps of:introducing a first precursor material into a chamber formed within afirst elongated tube, introducing a second precursor material into achamber formed between the first elongated tube and a second elongatedtube disposed concentrically about the first elongated tube, formingionized gas plasmas in the chambers to produce free radicals from theprecursor materials, and cooling the free radicals as they emerge fromthe chambers to promote combination of the free radicals to form thepredetermined chemical species.
 14. The method of claim 13 wherein theprecursor materials are selected to produce all of the free radicalsneeded to form the predetermined chemical species.